This invention relates to a sheet feeding device for a copying apparatus or a printing apparatus.
One type of sheet feeding device known in the art comprises a sheet feeding member operative to come into contact with the uppermost sheet of a stack of sheets, and sheet separating means disposed downstream of the sheet feeding member with respect to the sheet feeding direction for separating the uppermost sheet from the second and following sheets when a plurality of sheets are inadvertently fed by the sheet feeding member.
In most of the sheet feeding devices of the aforesaid type, the sheet separating means generally comprises a pair of separating members, one of them is constructed as a feeding member rotating in a direction in which the sheets are fed, and the other is constructed as a braking members rotating in a direction opposite to the sheet feeding direction. Generally, in the sheet separating means of this construction, the spacing between the feeding member and the braking member is determined in such a manner that the spacing allows only one sheet to pass therethrough when the sheet feeding speed is at a predetermined level. Also, the coefficient of friction of the separating members with respect to the sheets or the ratio of the peripheral velocity of one separating member to the peripheral velocity of the other separating member is selected such that the feeding member exerts a higher sheet conveying force than the sheet arresting force exerted by the braking member.
In the case of a copying apparatus of the diazo type, however, wherein the sheet conveying speed can be varied, the difference between the sheet conveying force exerted by the feeding member of the sheet feeding device and the sheet arresting force exerted by the braking member would increase when the sheet feeding speed becomes higher than the predetermined level. That is, as the velocity of the feeding member and the velocity of the braking member increase at the same rate, the difference between their velocities would become greater. With the spacing between the feeding member and the braking member being constant, this increase in the difference between the two velocities would result in an increase in the sheet feeding speed over the predetermined level of sheet feeding speed, thereby causing the sheet conveying force exerted by the feeding member to become inordinately higher than the sheet arresting force exerted by the braking member. Thus when a plurality of sheets are inadvertently fed by the sheet feeding member, the sheets would be fed without being separated by the sheet separating means. Conversely, when the sheet feeding speed became lower than the predetermined level, the difference between the sheet conveying force exerted by the feeding member and the sheet arresting force exerted by the braking member would become small, so that the sheet arresting force exerted by the braking member would become inordinately high as compared with the sheet conveying force exerted by the feeding member. Thus the sheets would stop at the sheet separating means and no sheets could be fed to the printing station.
The aforementioned disadvantages can be obviated if the spacing between the feeding member and the braking member is adjusted by means of an adjusting mechanism each time the sheet feeding speed is varied. However, spacing adjusting is a time-consuming operation because the adjusting mechanism of the prior art is manually operated and has to be fixed each time adjustment is effected.